Characterization and Mutational Analysis of Two UDP-Galactose 4-Epimerases in Streptococcus pneumoniae TIGR4

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L. L. Chen^1,2, D. L. Han¹, Y. F. Zhai¹, J. H. Wang², Y. F. Wang², and M. Chen^1*

¹State Key Laboratory of Microbial Technology, School of Life Sciences, Shandong University, Jinan, Shandong 250100, China; E-mail: chenmin@sdu.edu.cn

²Institute of Agro-Food Science and Technology, Shandong Academy of Agricultural Sciences/Key Laboratory of Agro-Products Processing Technology of Shandong Province/Key Laboratory of Novel Food Resources Processing, Ministry of Agriculture, Jinan 250100, China; E-mail: chenleilei8210@163.com

^* To whom correspondence should be addressed.

Received July 5, 2017; Revision received September 30, 2017

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Current clinical treatments for pneumococcal infections have many limitations and are faced with many challenges. New capsular polysaccharide structures must be explored to cope with diseases caused by different serotypes of Streptococcus pneumoniae. UDP-galactose 4-epimerase (GalE) is an essential enzyme involved in polysaccharide synthesis. It is an important virulence factor in many bacterial pathogens. In this study, we found that two genes (galEsp1 and galEsp2) are responsible for galactose metabolism in pathogenic S. pneumoniae TIGR4. Both GalESp1 and GalESp2 were shown to catalyze the epimerization of UDP-glucose (UDP-Glc)/UDP-galactose (UDP-Gal), but only GalESp2 was shown to catalyze the epimerization of UDP-N-acetylglucosamine (UDP-GlcNAc)/UDP-N-acetylgalactosamine (UDP-GalNAc). Interestingly, GalESp2 had 3-fold higher epimerase activity toward UDP-Glc/UDP-Gal than GalESp1. The biochemical properties of GalESp2 were studied. GalESp2 was stable over a wide range of temperatures, between 30 and 70°C, at pH 8.0. The K86G substitution caused GalESp2 to lose its epimerase activity toward UDP-Glc and UDP-Gal; however, substitution C300Y in GalESp2 resulted in only decreased activity toward UDP-GlcNAc and UDP-GalNAc. These results indicate that the Lys86 residue plays a critical role in the activity and substrate specificity of GalESp2.

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KEY WORDS: UDP-galactose 4-epimerase (GalE), Streptococcus pneumoniae TIGR4, mutation, substrate specificity

DOI: 10.1134/S0006297918010054

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